Method of separating highly adsorbable components in a gas stream in a pressure-sensing adsorber system

ABSTRACT

Pressure swing adsorption with increased recovery of a highly adsorbable product gas component is carried out with a cycling in which, during the flushing phase, and at least over the latter portion thereof, the flushing gas has a concentration in the product gas component which is substantially equal to that of the product gas. To further increase the yield and the concentration of the product gas component in the product gas, the pressure in the absorber is reduced over a time period between the termination of the adsorption phase and the beginning of the desorption phase, with the effluent gas during this reduction in pressure being reused in the pressure swing adsorption cycle.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is related to the commonly assigned copending applications Ser. Nos. 504,251 and 582,433 (now U.S. Pat. No. 4,521,221) filed 14 June 1983 and 22 Feb. 1984, respectively.

FIELD OF THE INVENTION

Our present invention relates to a method of separating one or more easily adsorbable components, e.g. methane or carbon dioxide, from a gas stream containing same having less readily adsorbable components (waste gas components) using a pressure-sensing adsorption technique. The invention is also, therefore, directed to a method of operating a pressure swing adsorber system and especially a pressure swing adsorber system utilizing adsorbers whose adsorption beds are comprised of carbon-containing materials. All of the principles of the invention are appropriate also to zeolites and other molecular sieve adsorbers.

BACKGROUND OF THE INVENTION

It is known that carbon dioxide, methane and other relatively highly adsorbable gas components (hereinafter also referred to as product gas components) can be separated from raw gas mixtures containing them in association with less readily adsorbable gas components, also referred to hereinafter as waste gas components, upon suitable adsorbers in appropriate beds. The adsorbers which are most commonly used are those which contain carbon, i.e. are active carbons.

In the pressure swing adsorption system, each adsorber is passed through a cycle in which the adsorber bed is charged with the highly adsorbable components in an adsorption phase at a first pressure. The bed is then flushed in a flushing phase with a flushing gas, and the product gas components are then desorbed by reducing the pressure in the adsorber to a desorption pressure during a desorption phase.

The pressure swings to which each adsorber may be subjected are generated at least in part by connecting one adsorber to another adsorber at the pressure which is desired in the first and allowing the two interconnected adsorbers to reach an equilibrium.

Until now it has been difficult, if not impossible, to recover these adsorbed product gas components in a pure form, i.e. in a concentration in excess of 99.5% by vol. by the desorption step in a satisfactory yield.

In various technological applications, these product gas components are available at concentrations between 15 and 50% by vol. in gas mixtures which makes their recovery feasible, especially if the product gas which is recovered can contain them in concentrations of 99.5% by vol. or more. Thus, it is advantageous to recover methane from mine gases, acetylene from acetylene/hydrogen circulating gases, carbon monoxide from a reformer gas and carbon dioxide from a converter or flue gases.

The invention is applicable to all of these separations and indeed any separation in which methane or carbon dioxide is to be recovered in high yield and in high concentration from a raw gas mixture.

We have discovered that one of the problems with earlier methods is that during the desorption stage, concurrently adsorbed other gas components, i.e. gas components other than the highly adsorbable desired product gas components, may be liberated to form impurities in the product gas mixtures which are similar to the impurities which may be present in the starting or raw gas mixture.

This problem has been attacked by a method which, as set forth in Ser. No. 504,251, which is not a reference in the present case, and which comprises steps (a)-(d) as outlined below.

OBJECTS OF THE INVENTION

It is the principal object of the present invention, therefore, to provide an improved method of operating a pressure swing adsorber system which will obviate disadvantages with earlier PSA systems and which will allow comparatively high outputs of product gas having a high concentration of the desired product gas component.

Another object of this invention is to provide an improved method of separating highly adsorbable product gas components from waste gas components which will extend the principles of the commonly assigned copending application Ser. No. 504,251 and the corresponding German application P 32 22 560.1 which are incorporated by reference herein.

Still another object of this invention is to provide an improved method of recovering methane and carbon dioxide in especially high yields and concentrations from raw gas containing same in amounts from below 15% by vol., for example, can be achieved without detriment to the yield.

SUMMARY OF THE INVENTION

These objects and others which will become apparent hereinafter are attained, in accordance with the present invention, in a method of recovering in a pressure swing adsorption (PSA) process, at least one relatively highly adsorbable product gas component (preferably carbon dioxide or methane) from a raw gas containing this product gas component in admixture with at least one adsorbable gas component in a PSA cycle wherein:

(a) in an adsorption phase in a pressure swing adsorber, the raw gas is passed through an adsorber bed which preferably contains carbon but may, as noted below, be a zeolitic or other molecular sieve bed to preferentially adsorb the highly adsorbable product gas component in a first gas flow direction, or the bed and pass a waste gas containing the less adsorbable component;

(b) the bed is flushed in a flushing phase with a flushing gas at substantially atmospheric pressure such that at least toward the end of the flushing phase the flushing gas has a concentration of said highly adsorbable product gas component substantially equal to that in a product gas;

(c) the pressure is thereafter reduced in the adsorber in a desorption phase to desorb the highly adsorbable product gas component from the adsorber to produce the aforementioned product gas containing the highly adsorbable product gas component, withdrawing the product gas from the adsorber, and feeding a portion of the product gas to the flushing gas for use in subsequent practice of step (b) (supra); and

(d) the pressure in the adsorber desorbed in step (c) is thereafter rebuilt at the adsorption pressure in a pressure-building phase. According to this invention, the foregoing method is improved by the steps of:

(e) intermittently between the end of the adsorption phase and before the commencement of the desorption phase, a partial gas flow is drawn from the adsorber in the aforementioned direction, i.e. in the same direction as the flow therethrough in the immediately prior period, to reduce the pressure in this adsorber; and

(f) reusing the partial gas flow in the pressure swing adsorption process.

It should be understood that the reference in the foregoing to a high adsorbable product gas component can be interpreted to mean a single gas or two or more highly adsorbable gases and if the latter is the case, that the concentration of all of the gases consisting of this component will be recovered in high concentration, i.e. 99.5% by vol. or more, in the product gas mixture, although none of these gases themselves may reach this high concentration. Furthermore while the method has been broadly described in connection with the recovery of methane and/or carbon dioxide from raw gas utilizing a carbon-containing adsorber bed (activated carbon), other adsorbers may be used, other product gas components can be recovered and other raw gas mixtures can be employed.

The invention is based upon the discovery that while the flushing phase between the adsorption phase and the desorption phase has generally been considered to be sufficient to free the interstices of the adsorber bed and the spaces within the adsorber from residual raw gas before desorption is commenced, this alone is not sufficient to ensure the especially high concentrations of product gas components in the product gas which the present invention seeks to obtain at high yields. Surprisingly, when there is a special pressure reduction phase, which need not be provided in every cycle but may be provided in each cycle if desired, during the period of the flushing phase and occupying a short fraction of this phase or, as more broadly stated, over an interval less than the entire flushing phase, the partial stream which is recovered in uniflow or codirectional flow with the previous passage of gas through the chamber, carries off contaminants from the product gas which would not have been removed in the flushing operation but would have been drawn into the product gas. Of course, a certain amount of product gas component is displaced along with the contaminants into this partial gas stream, but these product gas components are not lost to the process since the partial gas stream is recycled through the process or, put otherwise, is subjected to at least one further adsorption stage. The additional contaminants which are removed are those which appear to have been found formerly in the interstices of the adsorber or on the surface thereof to be dislodged by the flushing gas and are discharged in such quantities that the final desorption step yields a product gas of especially high concentration in the desired product gas components.

The special pressure reduction step can be effected prior to or subsequent to the flushing with a flushing gas having a product gas component concentration corresponding substantially to that in the product gas, i.e. prior or subsequent to flushing with the product gas.

Advantageously, this flushing phase is effected in uniflow with the adsorption phase, i.e. with the flushing gas traversing the adsorber bed in the same direction as the adsorption flow of gas during the adsorption phase.

It has been found to be advantageous to carry out the adsorption phase just prior to the beginning of breakthrough of the product gas components in the effluent from the adsorber bed. This mode has been found to be in symbiotic or synergistic relationship with the special reduction step in augmenting the degree of the enrichment of the product gas in highly adsorbable components.

According to a feature of the invention, the adsorption phase is carried out at a gas pressure which is significantly above that of ambient atmospheric pressure (at least 3 bar), a pressure at which the raw gas can be made available, e.g. as a product or waste gas from a chemical process.

This adsorption phase at elevated pressure can be preceded by a preadsorption stage at the same pressure in which preferably water vapor is removed from the raw gas so that the preadsorption step constitutes a drying stage.

After termination of the adsorption phase we can reduce the pressure to approximately atmospheric pressure and carry out at this pressure level the flushing phase with a gas of approximately product gas quality, at least toward the end of the flushing phase.

The adsorption at elevated pressure allows the adsorber volume to be minimized and permits the pressure reduction of a parallel working adsorber to the pressure above atmospheric pressure at least in part to be associated with the pressure reduction in another adsorber with the partial gas flow generated between adsorption and flushing phases in the manner described.

The special pressure reduction step can be carried out even in the case of an adsorption phase operating under atmospheric pressure as the adsorption phase pressure by application of a vacuum, the subsequent flushing phase being carried out again at atmospheric pressure.

Finally, the special pressure reduction step can also be carried out following the flushing phase by either equalizing the pressure in the adsorber which has been flushed with an evacuated adsorber which previously has undergone a product recovery or desorption phase, or by connecting the flushed adsorber to a vacuum pump. Product gas recovery thereafter follows at yet a lower pressure.

According to a feature of the invention, a gas is recovered during flushing in step (b) initially from the adsorber which is combined with the waste gas while during a subsequent generation of the flushing phase in step (b), e.g. when the flushing gas has a concentration of the product gas components close to that of the product gas, which is subjected to further enrichment in the product gas component by pressure swing adsorption. Best results are obtained when 10 to 90% by vol. of the recovered product gas is used in step (b) for flushing at least in the later stage of the flushing step of an adsorber.

In line with this feature of the invention, we have found it to be advantageous when, during substantially the last third of the duration of the flushing phase the resulting gas enriched in the product gas component is fed during a period or interval ranging from the end of an adsorption phase at the beginning of a flushing phase of a further adsorber operating in parallel to the first mentioned adsorber, to this further adsorber.

According to a feature of the invention, the pressure rebuilding in step (d) is carried out with the raw gas when the same gas is a gas mixture containing at most 15% by vol. carbon dioxide or methane.

It has been found to be advantageous, moreover, when the product recovery in step (c) is carried out with withdrawal of the product gas from opposite ends of the adsorber during the desorption stage. In particular, the pressure rebuilding in step (d) and the subsequent adsorption phase in step (a) should be carried out for a total time interval or period which ranges between half the duration of the others of the phases, namely, the desorption or flushing phases.

As will be described in greater detail below, the desorption phase (c) is best carried out in two stages, including a first stage in which the adsorber is connected to a first vacuum pump reducing the pressure in the adsorber to a pressure from that of flushing in step (b) to an intermediate pressure; and a second stage directly following the first stage in which the adsorber is connected to a second vacuum pump reducing the pressure in the adsorber from the intermediate pressure to a final desorption pressure at which the product gas is evacuated from the adsorber by the second vacuum pump. This intermediate pressure should be between 0.1 and 0.95 (preferably between 0.4 and 0.8) times the difference between the pressure of the flushing step (b) and the final desorption pressure. The final desorption pressure should be between 0.01 and 0.6 (preferably between 0.03 and 0.2) bar.

While, as mentioned, the flushing step (b) can be carried out at a pressure equal to that of the adsorption step (a), in one variant of the invention, the flushing phase is carried out at a pressure lower than that of the adsorption phase.

When this adsorption is carried out at a pressure greater than ambient atmospheric pressure, the gas evolved during the reduction in pressure to the flushing phase is discharged as waste gas as long as the concentration of the product gas component therein is less than that of the raw gas. However, when the gas evolved during this reduction in pressure in the adsorber from the adsorption phase pressure to the flushing phase pressure has a concentration of the product gas component which exceeds that of the raw gas, this gas released during the pressure reduction can be used to effect the pressure buildup in a desorbed reactor during a corresponding step (d).

The gas evolved during this reduction in pressure at the flushing phase pressure can be fed to a compressor for the same gas compressing the same at the adsorption phase pressure from the concentration of the product gas component in the evolved gas exceeding the concentration of the product gas component in the raw gas or at least equaling same.

Further reduction in the pressure in the adsorber from the adsorption phase pressure is terminated and flushing is then carried out in step (b) at the termination pressure when the product gas component concentration in the gas evolved during the reduction in pressure has a value between the mean concentration of the product gas component in the product gas and the concentration of the product gas component in the raw gas.

Unless otherwise stated, where reference is made herein to the concentration of the product gas component, the concentration in volume % is intended.

The product gas components with which the invention is concerned, are those which are relative with respect to all of the components of a gas mixture, the most strongly adsorbed on the particular adsorbent used and thus the components which tend to concentrate on the upstream side of the adsorption bed, the less strongly adsorbed components by those which flush through the bed. It is these less strongly adsorbed components which have been referred to as waste gas components. Naturally, when such a weakly adsorbed component is a desired product, it may also be recovered in high concentration and relatively high yield because of the increased removal of the more highly adsorbable components. In other words, just as the product gas constitutes a highly enriched state with respect to the product gas components, the waste gas can be considered a highly enriched state of the remaining component or components.

The flushing phase is carried out with a gas mixture generated during the pressure swing process and which is enriched (by comparison with the raw gas) in the product gas component or components. Under the concentration of the product gas component in the flushing gas can continuously be increased or can be increased in a stepwise manner as the flushing proceeds as long as at least toward the end of the flushing phase the flushing is carried out with a gas having substantially product gas quality. Thish flushing step, which as a rule is carried out at atmospheric pressure, at least when the adsorption phase is carried out at atmospheric pressure appears to drive out residual gas or less adsorbable components from the interstices of the adsorption bed and from the adsorber so that free volume in the adsorber is replaced by gas of product like quality or at least so enriched with respect to the product gas component, that practically no raw gas or less adsorbable residues are entrained during the desorption phase in the product gas. As a practical matter, therefore, at the end of the flushing phase, not only is the adsorbent substantially saturated with the product gas component which is to be recovered, especially if adsorption is terminated just prior to breakthrough of this component, but the interstices of the adsorber bed and any free space in the adsorber are likewise filled with the product gas component at least in its concentration of the product gas mixture. Only negligible amounts of impurities are thus found in the product gases recovered by evacuating or dropping the pressure in the adsorber for the desorption phase.

The flushing, of course, should be carried out until the concentration of the product gas component in the flushing stream downstream of the adsorber has the flushing concentration of that of the product gas so that this effluent flushing stream can be considered 100% product gas. Practically in all cases, however, the flushing gas will have a concentration of the product gas component above that of the raw gas so that this gas is practically suitable for reuse in the pressure swing adsorption process as a source of the product gas component. At least toward the end of the flushing phase of one adsorber, the gas effluent therefrom can be used with excellent effect in the adsorption or flushing phases of a parallel operating adsorber because the gas concentration can be rapidly enriched to product gas quality and thus this gas is especially effective as a source of the product gas component or as a flushing gas.

It is self-understood that in start-up of an apparatus or plant for practicing the method of the invention a certain start-up time is required until the flows, pressures and gas concentrations are available to practice the invention in accordance with the present principles so that at least a number of cycles is required until the operational steady state is reached.

One of the features of the invention is that the adsorption bed in the method of the invention can be restored rapidly and almost completely to its original adsorption capacity in each cycle by carrying out the fullest possible desorption and by maintaining all of the phases of the pressure swing process substantially adiabatic in nature.

The process of the invention is applicable to all relatively strongly adsorbable gases, especially methane and carbon dioxide and hence to raw gases containing same and thus to gas mixtures which also can include oxygen, nitrogen, carbon monoxide, hydrogen, helium, argon and the like. The latter gases are all components which are less readily adsorbed from carbon-containing adsorbents such as active carbon, a preferred adsorbent. With these, the product gas is obtained in especially high purity and yield.

Surprisingly, the process is not adversely affected when the starting mixture, i.e. the raw gas, also includes traces (for example less than 1% by vol.) of gas components which are even more strongly adsorbed than the product gas components. Since these components are not released in the special pressure reduction step in accordance with the invention, the adsorber bed is always preferentially charged with the product gas component and this impurity does not enter the product gas in substantial amounts. The more strongly adsorbed trace component can be removed following the desorption step utilizing a flushing operation if desired. Alternatively, this component can remain in the adsorbent as a contaminant, being liberated only upon regeneration of the adsorbent after a long series of adsorption/desorption cycles of the particular bed.

If there is a danger that tolerance limits for such a component may be exceeded, a special prefilter can be utilized to recover or remove such trace impurities before they enter the adsorber with the same gas. For example, we may provide a dryer for removing water vapor from the raw gas. Naturally the adsorber itself must have the requisite adsorption characteristics with respect to the product gas components which are to be recovered.

The process of the invention can also be used, for example, for the recovery from nitrogen from air utilizing zeolites or other molecular sieve adsorbers since nitrogen tends to be more readily adsorbed than oxygen on zeolites and in the pressure swing adsorption system of the invention.

Naturally, even prior to this invention, it was known to separate gas mixtures using the different adsorption and desorption characteristics of the gas components on adsorbents with pressure swing or temperature swing techniques. The invention, however, provides a method which distinguishes over these earlier systems practically by ensuring that in the adsorber, at the beginning of the desorption phase, there is practically no other gas components present besides the highly adsorbable product gas components desired in the product gas which is to be produced.

The practically complete charging of the adsorber with the product gas component is achieved in accordance with the principles of the invention by carrying out the flushing following the adsorption phase with a portion of the product gas which already has a concentration in excess of 99.5% by vol. product gas components so that any adsorption residues of raw gas are eliminated and indeed any residues from the flushing stage in the adsorber have essentially the same concentration of product gas components as the product gas itself. Any gases with a concentration of product gas components between the product gas concentration and the starting or raw gas concentration are recycled to the process or are treated anew in the pressure swing process.

Indeed, as noted earlier, at least 10% of the product gas produced and up to 90% of this product gas is utilized as the flushing gas at least during the later stages of the flushing phase. The specific proportion of the product gas which is used in this manner will of course depend upon the concentration of the product gas components in the starting or raw gas mixture. The lower the reduction gas component concentration in this mixture, the higher must be the proportion of the product gas which is utilized as the flushing gas.

Advantageously, during the flushing of the adsorber with product gas, the gas which leaves the adsorber during the first two thirds of the flushing can be discharged into the waste gas and only the gas which emerges during the last third of the flushing process can be used in a recycle to be subjected to renew adsorption or flushing. The recycling time can thereby be reduced with increased output or yield.

We can also practice the invention so that the flushing is terminated before complete breakthrough of product gas components is achieved, i.e. before the product gas concentration at the effluent end of the adsorber bed or the adsorber during flushing exceeds the concentration of product gas components in the flushing gas or, where the adsorption has been terminated sufficiently ahead of breakthrough of the product gas components, before product gas components emerge in the effluent gas during flushing.

In this case, the gas mixture recovered at the outset of the desorption phase following the flushing of a particular adsorber can be used as the initial flushing gas feed to another adsorber and only after a certain time can the product gas from the desorption phase, in which the concentration of product gas components is in excess of 99.5% by vol., be recovered for use in part as a later stage flushing gas and in part as the product which is withdrawn from the system.

In addition, for the desired complete charging of the adsorber with the product gas components prior to desorption, it has been found to be advantageous to effect the pressure buildup in the case when the starting gases contain less than about 15% by vol. carbon dioxide or less than 10% by vol. methane with the starting gas itself. With raw gases with higher concentrations of the product gas components, however, the pressure buildup can be effected with the waste gas from the separation of the present invention. According to another preferred feature of the invention during the adsorption phase following the pressure buildup and before the admission of the raw gas to be separated, a portion of a gas effluent from a flushing phase of another cycle or a gas containing an enriched concentration of the product gas component from another source or a combination of the two, can be admitted during the adsorption phase. These approaches are advantageous in ensuring, upon the evacuation of the adsorber in the desorption stage, the highest yield of a product enriched to at lest 99.5% by vol. with the product gas component.

The flushing of the adsorber should be effected in uniflow or concurrent flow, both of these terms being used to indicate the same direction of flow as in a previous stage, in this case the adsorption stage. The desorption, however, can be effected either in the uniflow or in the counterflow direction or, as previously mentioned, a combination of both with the product gas being evacuated simultaneously from the opposite ends of the adsorber.

The durations of the adsorption and desorption phases should be so selected that the pressure buildup charging of the adsorbent with the product gas component in the adsorption phase should be between half to twice as long as each or either of the flushing and desorption phases. For example, when the adsorption phase is about twice as long as the flushing or desorption phase in a four-adsorber system, optimum results are obtained.

For a three-adsorber pressure swing system it suffices that the duration of the combination of the pressure buildup and adsorption phase, the flushing phase, and the desorption phase be effected for substantially equal time periods. However, this results in a somewhat reduced efficiency by comparison with the four-adsorber system.

For the flushing of an adsorber a relatively high volume of gas is required and this must be recovered in accordance with the principles of the invention, by evacuation of another adsorber. This means that gas evacuated from one adsorber is only in part used as the product gas while the remainder is utilized for flushing of a second adsorber.

The energetics or energy efficiency of a system according to the invention depends upon the pump energy utilized for such evacuation. This will, of course, also depend upon the proportion of the evacuated gas which is utilized as the product gas and which is recycled as the flushing gas, i.e. the less flushing gas is recycled compared to the non-recycled portion of the product gas, the lower is the energy consumption and the higher the efficiency.

We have found that it is possible to minimize the energy consumption by carrying out the invention in various ways with respect to the evacuation of the adsorber during the desorption phase.

Especially advantageous is the provision of two vacuum pumps or vacuum pump sets (version 1) of which the first is utilized to reduce the pressure from the flushing pressure to an intermediate pressure, i.e. the intermediate pressure mentioned previously. In this case the second vacuum pump is utilized to reduce the pressure from the intermediate pressure to the final desorption or evacuation pressure. The first pump, therefore, is utilized to displace the flushing gas exclusively. Product gas is drawn off by the second pump. Naturally, the pumps are proportioned to provide the desired ratio of the flushing gas to the extracted product gas.

Tests have shown that it is especially advantageous to utilize each pump only during the respective evacuation process for the respective pressure range with each pressure range for the respective pump by less than the entire pressure range or gradient between the flushing pressure and the final pressure. During the first evacuation step or stage, therefore, relatively a large quantity of gas is evolved with a comparatively small pressure gradient or differential (during the first gas generating stage) whereas during the second evacuation stage, a comparatively small volume of gas is withdrawn with a comparatively large pressure differential or gradient.

Since these pumping characteristics are quite different in the two stages, we can make use of the respective pumps which operate most efficiently for the respective stage, i.e. a high volume low pressure pump for the first stage and a low volume high pressure pump for the second stage. Apart from the improvement in energy utilization, the use of two distinct pumps has been found to give rise to unexpected advantages since it has been found heretofore that desorption under reduced pressure generally requires more than is theoretically required (cf. E. Richter et al, Chemie-Ingenieur-Technik 50, 1978, pp 600-611).

This considered the desorption as the inverse of adsorption whose velocity is determined exclusively by the diffusion of the gas in the pores of the adsorbent.

The further advantage of the use of two vacuum pumps is found in the fact that it is not essential to utilize for flushing an adsorber the gas which has precisely the product gas quality since the first vacuum pump can supply a gas which has close to product gas quality for this purpose. This can reduce the quality of flushing gas which is required without detriment to the product gas. Another important advantage is that the efficiency of the pressure swing plant is less effected by intermittent variations in the raw gas composition, this being compensated by simply changing the proportions of the gases which are used as the flushing gas and as the product gas by varying the proportions drawn from the adsorber by the two vacuum pumps. The two pumps are less apt to experience pressure jumps and the values of the gases extracted by the two pumps seldom will vary significantly at least in a sudden manner. Indeed, in this variant a lower final pressure can be generated with a larger evacuation time without special pump structures, thereby increasing the output of the plant. In many cases, the raw gas is made available at a pressure substantially greater than ambient pressure. In this case it is not possible to utilize the gas resulting from the evacuation of an adsorber directly for a flushing of the adsorber to the adsorption pressure level. The reason is that the vacuum pump which is used generally has limited capacity to displace the gas against a superatmospheric pressure.

In this case an additional compressor is required with higher capital cost and operating cost. To reduce the charging pressure is possible but we have found it to be especially advantageous to carry out the charging of the adsorber at the elevated pressure at which the gas becomes available and to effect the flushing, not at this superatmospheric adsorption pressure, but rather at a pressure at which the flushing gas becomes available from its vacuum pump. This applies not only to the system utilizing two vacuum pumps in the manner described, but also to a system in which one vacuum pump is used to evacuate the adsorber in the desorption phase and the outflow from the vacuum pump is split between the flushing gas stream and the product gas stream. This has been found to be advantageous in a mode according to the invention (version 4 described more fully below) wherein after charging of the adsorber, the latter is expanded in uniflow or codirectional flow.

This expansion has the effect of immediately increasing the enrichment of the product components in the adsorber with respect to the other components.

Tests have surprisingly shown that losses which might be expected in the product gas from such an expansion can be avoided when the charging of the adsorber with the raw gas is interrupted before the breakthrough or advance front of the product gas components has reached the downstream side of the adsorber bed and when the expansion is effected in uniflow as this has been defined above after termination of the adsorption phase and before breakthrough of the product gas components.

In this mode of operation, a reduced flushing gas value is required.

Unexpectedly it has been found further that following a charging of the adsorber at a pressure which is substantially greater than ambient pressure, it is possible to minimize the flushing gas value required when a pressure equalization is effected between an adsorber whose evacuation has terminated immediately prior thereto and yet is at the final pressure of evacuation (version 3) and the adsorber in which adsorption has just terminated. This has been found to be especially advantageous in reducing gas volume needed for the pressure rebuilding. This is important most specifically when the product gas components are already in comparatively high concentrations (for example concentrations in excess of 60% by vol.) in the feed or raw gas. A reduction in the flushing gas quantity can be ensured also when the pressure equalization follows flushing (version 2). This is also observed when the charging is effected at high pressure and is followed by a uniflow expansion.

BRIEF DESCRIPTION OF THE DRAWING

The above and other objects, features and advantages of the present invention will become more readily apparent from the following description, reference being made to the accompanying drawing, in which:

FIG. 1 is a diagram illustrating a four-adsorber plant for carrying out the process of the invention in accordance with version 1 thereof;

FIG. 2 is a timing diagram graphically illustrating the operation phases of the four adsorbers in version 1 in FIG. 1;

FIG. 3 is a diagram showing the connection of four adsorbers to effect the method of version 2 of the invention;

FIG. 4 is a graphic depiction of the timing characteristics of the latter method;

FIG. 5 is a diagram similar to FIG. 3 illustrating a four-adsorber plant connected so as to carry out version 3 of the invention;

FIG. 6 is a timing diagram for this arrangement of the adsorbers;

FIG. 7 is a connection diagram for the four-adsorbers used in carrying out version 4; and

FIG. 8 is a graphic depiction of the timing of the operation of this use of the adsorbers.

SPECIFIC DESCRIPTION AND EXAMPLES

In the description below, the plant shown in FIGS. 1, 3, 5 and 7 will be described as to the respective connection in their applications for carrying out versions 1, 2, 3 and 4 representing the specific examples.

Version 1

In this version, two vacuum pumps are provided, one of which serves to provide the flushing gas while the second withdraws the product gas. The separation of these two functions as to the respective vacuum pump has the important advantage that each of the pumps supplies a respective function and can be dimensioned for the characteristics for the particular functions as described. The vacuum pump which is utilized is utilized to generate the flushing gas if required to draw a relatively reduced vacuum and thus will have a similar pressure reduction capacity than the other pump which must be capable of drawing a deeper vacuum.

The plant utilizing this pump has been shown in FIG. 1 and the timing diagram is represented in FIG. 2.

At the bottom of FIG. 2, we have plotted the pressure during a single cycle in each of the phase shifted adsorbers 1-4, the pressure being given along the ordinate. Naturally this also represents a graph of the pressure in a single adsorber in which time would be plotted along the abscissa.

The incoming raw gas is assumed to be at a pressure slightly above ambient atmospheric pressure and sufficient to overcome pressure losses in the adsorbers and in the pressure system supplying same. The product gas is recovered at a pressure which depending upon the characteristics of the vacuum pump may be more or less above the ambient atmospheric pressure. The following description is applied to a single adsorber and sets out the sequence of phases thereof for a complete pressure swing cycle, it being noted that, apart from evacuation to recover the product flushing gases from an adsorber, the pressure variations in each adsorber are obtained by connecting that adsorber to another adsorber or to the source of raw gas.

The raw gas flows, e.g. via the valve I and line L through the open valve 21 (resp. 22 or 23 or 24) into the adsorber 1 (resp. 2 or 3 or 4) which has previously been brought to the adsorption pressure which it slightly exceeds atmospheric pressure at the time t₁ by a pressure rebuilding step commencing at t₀ by connection to another adsorber.

The adsorber 1, like adsorbers 2, 3 and 4, can be charged with a carbon-containing adsorbent, e.g. active carbon. The waste gas leaves via valve 17 (resp. 18 or 19 or 20) and can be discharged via valves 26 and 25 through the waste gas line W although it can be recycled in part through another set of valves, not shown, for the pressure building phase.

During the charging step, the adsorbent is charged with the product gas component, e.g. methane or carbon dioxide from the upstream end 1a to the downstream end 1b of the adsorbent, i.e. in the flow direction 1c. Prior to breakthrough of the product gas component at the downstream end, i.e. before highly adsorbable gas in noticeable concentrations leaves the adsorber, the charging is interrupted. This is represented at point t₂. It may be assumed that a portion of the waste gas has been conducted via valve 18 (resp. 19 or 20 or 17) to the adsorber 2 (resp. 3 or 4 or 5) for pressure buildup in the latter.

Following the charging of the adsorber, the adsorber 1 (resp. 2 or 3 or 4) is flushed with a gas which is more substantially enriched with the strongly adsorbable product gas components in than the same gas. This can be effected with valve (resp. 9 or 10 or 11) opened from the adsorber 4 (resp. 1 or 2 or 3) undergoing desorption and circulating the gas mixture extracted via the first vacuum pump 27 and valve 5 (resp. 6 or 7 or 8) to the adsorber 1, this product gas recirculating as flushing gas approximating the product gas concentration in the more highly adsorbed component.

During the flushing step the gas is discharged from the adsorber via valve 13 (resp. 14 or 15 or 16) which is practically free from the strongly adsorbed components, i.e. is in effect raw gas from which the product gas component has been adsorbed and which is displaced from the adsorber bed and the free spaces in the adsorber housing or chamber. The adsorber itself is thereby so enriched with the product gas component that in the subsequent stage with reduction of the pressure at t₃, the gas evolved is highly enriched in the product gas component and is practically free from contaminants. Until the time t₄, this gas is utilized as a flushing gas for the next adsorber 2, the product gas being evolved at the final pressure P_(F) until the desorption is terminated at t₅ whereupon the cycle is repeated.

For the first phase of the desorption process a valve 9 (resp. 10 or 11 or 12) is opened and the compressor 27 recycles the flushing gas via valve 6 (resp. 7 or 8 or 5) to the next adsorber 2 (resp. 3 or 4 or 1). Between the times t₄ and t₅, valve 13 (resp. 14 or 15 or 16) is opened and the product gas is recovered via vacuum pump 28 and valve 29.

A pressure reduction is effected at P_(R) between the end of the adsorption phase at t₂ and before the beginning of the desorption phase at t₃, e.g. by connection of the adsorber 1 to the adsorber 4 or the adsorber 3 having a reduced pressure therein for the special pressure reduction step in accordance with this invention.

    ______________________________________                                         TEST DATA                                                                      ______________________________________                                         Adsorption means                                                                              carbon molecular sieve with a BET                                              surface of 1100 m.sup.2 /g                                      charging (adsorption)                                                                         1.05 bar                                                        pressure                                                                       raw gas        16.7% by vol. CO.sub.2                                                         83.3% by vol. N.sub.2                                           adsorber volume                                                                               1 m.sup.3                                                       mode of operation                                                                             version 1                                                       ______________________________________                                         gas volumes (normalized at standard temperature and pressure)                                     CO.sub.2 (m.sup.3)                                                                       N.sub.2 (m.sup.3)                                                                    total gas (m.sup.3)                         ______________________________________                                         1.  pressure buildup                                                                              0.25       1.32  1.57                                           (raw gas)                                                                  2.  adsorption     3.25      16.94 20.19                                           (raw gas)                                                                  3.  flushing       9.57       0.57 10.14                                           (product gas 1)                                                            4.  desorption     3.19      --     3.19                                           (product gas 2)                                                                waste gas:     0.31      18.26 18.57                                           (1.67% by vol. CO.sub.2)                                                   yield = (product gas volume) : (raw gas volume) = 0.91                         ______________________________________                                    

In FIG. 2 as in FIGS. 4, 6 and 8, the markings indicate the valves which are open for the sequencing of the adsorbers for the respective adsorption phases, thereby indicating the staggering of the adsorption phases.

Version 2

This version utilizes a single vacuum pump 160. Charging is effected at a pressure which is approximately ambient pressure and the differential pressure with respect to the waste gas pressure is such that it corresponds to the pressure drop or losses in the apparatus during adsorber charging. The flushing pressure is substantially equal to the adsorption pressure and pressure equalization is used in the manner described so that the vacuum pump 160 is not subjected to pressure jumps in switching from one adsorber to the next. The product gas recovery is practically constant in time. Here the pressure reduction between desorption and adsorption is commenced at T_(R), the desorption at t₃ and the pressure building is commenced at t₀.

The function of the apparatus of FIG. 3 will be described in terms of one pressure swing cycle of adsorber 101, the other adsorbers 102, 103 and 104 being staggered with respect thereto as represented by the diagram of FIG. 4.

The raw gas flows during the charging phase via the raw gas pipe 105 and the valve 111 (resp. 121 or 131 or 141) into the adsorber 101 (resp. 102 or 103 or 104) to charge the adsorber bed thereof with the highly adsorbable component in the manner described. The waste gas, substantially completely freed from this strongly adsorbable component flows via valve 115 (resp. 125 or 135 or 145) into the waste gas line 110.

Adsorption or charging of the adsorbent is interrupted at a predetermied time which may be established empirically and is selected such that the strongly adsorbable gas is not found in any material concentration in the waste gas.

At this point the adsorber 101 (resp. 102 or 103 or 104) is flushed with a gas which is drawn out of the adsorber 104 (resp. 101 or 102 or 103) by the vacuum pump 160 and, under the control of valve 150, 150a establishng the proportions of the product gas which is recycled as flushing gas, is fed to the adsorber 101. During this flushing phase, valves 143 (resp. 113 or 123 or 133), 112 (resp. 122 or 132 or 142) and 115 (resp. 125 or 135 or 145) are open.

The gas used for flushing purposes has practically product gas quality when it enters the adsorber 101 (resp. 102 or 103 or 104).

During flushing a gas leaves the adsorber 101 (resp. 102 or 103 or 104) which has a substantially reduced concentration of the strongly adsorbable component and, indeed, a concentration in this component which is less than that of the raw gas.

After connection of the flushing step at a predetermined period selected, likewise empirically, so that there is practically no breakthrough of the product gas component in the effluent during the flushing stage, the pressure in the adsorber is dropped in two stages (see FIG. 4). In the first stage, representing the special pressure reduction prior to desorption, the pressure drop is effected pressure equalization between the evacuated adsorber 104 (resp. 101 or 102 or 103) and the adsorber 101 (resp. 102 or 103 or 104) via the open valves 116 (resp. 126 or 136 or 146) and 147 (resp. 117 or 127 or 137).

In this case, gas flows via line 109 from adsorber 101 (resp. 102 or 103 or 104) into adsorber 104 (resp. 101 or 102 or 103) to provide the pressure rebuilding at time t0 in the latter adsorber. This gas has a concentration of the strongly adsorbable component which on the average is greater than that of the raw gas. After a predetermined time interval represented by the period between T_(R) and t₃, this pressure reduction is interrupted. This point corresponds to pressure equalization between the two adsorbers, the pressure being an intermediate pressure P_(I) between the raw gas pressure and the final desorption pressure P_(F).

In the next step, the adsorber 101 (resp. 102 or 103 or 104) is connected by the valve 113 (resp. 123 or 133 or 143) via line 106 with the vacuum pump 160 and the pressure is reduced commencing at the time t₃ to the final pressure P_(F) at a time t₄, this pressure being maintained until the desorption phase is complete.

The gas then evacuated from the adsorber has the product gas quality and is partly discharged as the product via line 106 and partly recirculated via line 107 and valve 122 (resp. 132 or 142 or 112) as the flushing gas to the adsorber 102 (resp. 103 or 104 or 101) it being operated in the adsorption phase.

Upon conclusion of the cycle at t₅ for the adsorber 101 (resp. 102 or 103 or 104), the pressure is rebuilt therein to the adsorption pressure in two stages. In the first stage a pressure equalization is effected with adsorber 102 (resp. 103 or 104 or 101) by opening the valve 117 (resp. 127 or 137 or 147) and 126 (resp. 136 or 146 or 116) via line 109. This corresponds to the establishment of a pressure t₁ ', intermediate the final pressure and the adsorption pressure t₁. In the second stage, raw gas is admitted through open valve 111 (resp. 121 or 131 or 141) into the adsorber 101 (resp. 102 or 103 or 104) to raise the pressure from the intermediate pressure t₁ ', to the adsorption pressure t₁. In the latter step the pressure is restored to the adsorption pressure and the cycle can be repeated for the adsorber. The other adsorbers are likewise operated in the staggered relationship described and represented in FIG. 4.

    ______________________________________                                         TEST DATA                                                                      ______________________________________                                         Adsorption means                                                                              carbon molecular sieve with a BET                                              surface of 1100 m.sup.2 /g                                      charging (adsorption)                                                                         1.05 bar                                                        pressure                                                                       raw gas        16.7% by vol. CO.sub.2                                                         83.3% by vol. N.sub.2                                           adsorber volume                                                                               1 m.sup.3                                                       mode of operation                                                                             version 2                                                       ______________________________________                                         gas volumes (normalized at standard temperature and pressure)                                     CO.sub.2 (m.sup.3)                                                                       N.sub.2 (m.sup.3)                                                                    total gas (m.sup.3)                         ______________________________________                                         1.  pressure buildup                                                                              0.29       1.45  1.74                                           (raw gas)                                                                  2.  adsorption     3.79      18.90 22.69                                           (raw gas)                                                                  3.  flushing       10.67     --    10.67                                           (product gas)                                                              4.  desorption     14.20     --    14.20                                           (product gas)                                                                  product gas yield                                                                             3.53      --     3.53                                           (99.9% by vol. CO.sub.2)                                                       waste gas:     0.55      20.35 20.90                                           (2.63% by vol. CO.sub.2)                                                   yield = (product gas volume) : (raw gas volume) = 0.87                         ______________________________________                                    

Version 3

This version also uses only a single vacuum pump and indeed a layout of the adsorber and the piping system generally similar to that of FIG. 3 being additionally equipped with a compressor 250 for raising the pressure of the raw gas to the adsorption pressure. In this embodiment, after the termination of the flushing phase, the concentration of the more strongly adsorbed component in the adsorber can be increased still further, so that less flushing gas is required or a gas of higher purity or concentration in this highly adsorbable component can be recovered. In this embodiment as well the vacuum pump 260 is subjected to less substantial pressure jumps. During switchover from one phase to another, at the beginning of evacuation, a pressure is supplied which is less than the raw gas pressure.

This apparatus will be described in connection with FIG. 5 and the valve control diagram shown in FIG. 6 at the bottom of which the pressure/time graph for a single cycle has been illustrated.

The raw gas is here available at a pressure which slightly exceeds ambient pressure, the pressure corresponding to the pressure drop in the apparatus between the inlet pipe 251 and the outlet or waste pipe 250 so that this pressure drop corresponds to the pressure losses until the waste gas is released into the atmosphere. The determination of this pressure is not however critical for the process as long as the pressure is selected so that the waste gas thereby released is at ambient pressure.

The product gas is recovered at a pressure which, depending upon the vacuum pump 260, can be more or less than ambient pressure. The distinction applies to a full cycle of the adsorber 201, the adsorbers 202-204 being similarly cycled in time offset to one another and at the adsorber 201. The raw gas is supplied via line 251 at the pressure established by the compressor 250 at sufficiently ambient pressure to compensate for the pressure drop described previously into the adsorber 201 (resp. 202 or 203 or 204) via the open valve 211 (resp. 221 or 231 or 241) in which the adsorption pressure corresponding to the raw gas pressure prevails.

The strongly adsorbable component is thus adsorbed from this gas so that the effluent gas discharged by valve 215 (resp. 225 or 235 or 245) into line 258 is substantially freed from the highly adsorbable component.

The adsorption is continued at a predetermined point immediately prior to breakthrough of the highly adsorbable component, i.e. to a point empirically determined and constituting a point just before the concentration of the highly adsorbable component noticeably increases in the effluent during the adsorption phase.

Thereafter the adsorber is flushed with the gas which is recovered by evacuation of the adsorber 204 (resp. 201 or 202 or 203). To this end the valve 242 (resp. 212 or 222 or 232) of the adsorber 204 (resp. 201 or 202 or 203) is opened. The product gas is withdrawn from the latter adsorber via the vacuum pump 206 and is supplied via the throttle valve 259 and the open valve 213 (resp. 223 or 233 or 243) to the adsorber 201 (resp. 202 or 203 or 204). During this phase, via the open valve 215, a flushing effluent is discharged into the waste gas line 258 with a concentration of the strongly adsorbable component which is less than that of the raw gas. Following termination of this flushing phase, which is carried out at the raw gas pressure, a pressure equalization is effected (special pressure reduction stage) by pressure equalization via line 253 between the adsorber 201 (resp. 202 or 203 or 204) and the previously evacuated adsorber 203 (resp. 204 or 201 or 202) by the opening of the valves 216 (resp. 226 or 236 or 246), and 234 (resp. 244 or 214 or 224).

During this pressure equalization, a gas from the adsorber 201 (resp. 202 or 203 or 204) is drawn into the adsorber 203 (resp. 204 or 201 or 202) whose concentration as to the strongly adsorbable component is also substantially below the raw gas concentration.

In the next stage or phase, the adsorber 201 (resp. 202 or 203 or 204) is evacuated from the intermediate pressure P_(I) which thus has resulted ato the final pressure P_(F) by opening valve 212 (resp. 222 or 232 or 242) which connects the vacuum pump 260 and the line 252 to the adsorber 201 (resp. 202 or 203 or 204). The product gas is partly discharged via the product gas line 261.

A portion of the gas during this evacuation of the adsorber 201 (resp. 202 or 203 or 204) is utilized to flush the adsorber 202 (resp. 203 or 204 or 201), valves 223 (resp. 233 or 243 or 213) and 225 (resp. 235 or 245 or 215) of this adsorber being opened for this purpose.

During the evacuation the presure in adsorber 201 (resp. 202 or 203 or 204) can be reduced to and maintained at the final pressure which may range between 10 and 600 mbar and can be determined by the characteristics of the pump 260.

After termination of the evacuation, following a brief dead time, pressure can be rebuilt in the adsorber 201 (resp. 202 or 203 or 204) in two stages. In the first stage the pressure is built up to an individual level by pressure equalization between the adsorber 201 (resp. 202 or 203 or 204) and the adsorber 203 (resp. 204 or 201 or 202) with valves 213 (resp. 233 or 233 or 243) and 236 (resp. 246 or 216 or 226) open. During the second stage of the pressure buildup, valve 211 (resp. 221 or 231 or 241) is opened to allow the raw gas from line 251 to raise the pressure in adsorber 201 (resp. 202 or 203 or 204) so that the adsorption phase can be begun anew.

    ______________________________________                                         TEST DATA                                                                      ______________________________________                                         Adsorption means                                                                              carbon molecular sieve with a BET                                              surface of 1100 m.sup.2 /g                                      charging (adsorption)                                                                         1.05 bar                                                        pressure                                                                       raw gas        16.7% by vol. CO.sub.2                                                         83.3% by vol. N.sub.2                                           adsorber volume                                                                               1 m.sup.3                                                       mode of operation                                                                             version 3                                                       ______________________________________                                         gas volumes (normalized at standard temperature and pressure)                                     CO.sub.2 (m.sup.3)                                                                       N.sub.2 (m.sup.3)                                                                    total gas (m.sup.3)                         ______________________________________                                         1.  pressure buildup                                                                              0.32       1.60  1.92                                           (raw gas)                                                                  2.  adsorption     3.99      19.90 23.89                                           (raw gas)                                                                  3.  flushing       8.07      --     8.07                                           (product gas)                                                              4.  desorption     12.18     --    12.18                                           (product gas)                                                                  product gas yield                                                                             4.11      --     4.11                                           (99.9% by vol. CO.sub.2)                                                       waste gas:     0.20      21.50 21.70                                           (0.90% by vol. CO.sub.2)                                                   yield = (product gas volume) : (raw gas volume) = 0.95                         ______________________________________                                    

Version 4

In this version we operate with a single vacuum pump 357. The charging or adsorption is here effected at an elevated pressure, i.e. one that can be well above ambient pressure, it being assumed that the raw gas is already available at an elevated pressure or is compressed 350 at this pressure by a compressor at the upstream side of the adsorption plant.

Adsorption at elevated pressure has the important advantage that the size of the adsorber can be reduced in that more gas per unit volume of adsorbent can be adsorbed. Furthermore, the use of the substantial adsorption pressure permits the subsequent expansion step to be effected in uniflow at the adsorption with further enrichment of the strongly adsorbed component in the adsorber so that the flushing steps as provided in versions 1-3 can be replaced by a flushing step at atmospheric pressure. A flow diagram of the plant connected for this version has been shown in FIG. 7 and the FIG. 8 represents the timing diagrams, the open valves being represented by crosses in appropriate boxes. At the bottom of the diagram of FIG. 8, we have graphed a pressure versus time relationship for the single adsorber 301.

As noted, the raw gas is supplied to a pressure which is substantially in excess of atmospheric pressure, preferably in excess of 2 bar, the diagram of FIG. 8 merely illustrating this by a pressure above 1 bar during the adsorption phase. It is assumed that this pressure is the pressure at which the raw gas becomes available from a chemical process. Of course, it may also be the pressure generated by the compressor 350.

The product gas is obtained, depending upon the characteristics of the vacuum pump 357, at a pressure which may be more or less above ambient atmospheric pressure.

While the cycle is described only for the adsorber 301, it will be understood that what is said with respect to this adsorber applies to the adsorber 302-304 as well, these adsorbers being staggered in operational sequence.

The raw gas during the charging or adsorption phase is fed via line 351 to the valve 311 (resp. 321 or 331 or 341) into the adsorber 301 (resp. 302 or 303 or 304) and the waste gas is discharged via valve 315 (resp. 325 or 335 or 345) and the line 359 which can include a throttle valve 360. The latter has the function of maintaining the pressure in the adsorbent constant during the adsorption phase.

At some time prior to a sharp increase in the concentration of the highly adsorbable product component in the waste gas, the adsorption phase is terminated. The adsorption is followed by the spherical pressure reduction which occurs at time T_(R) and involves an expansion of the adsorber 301 (resp. 302 or 303 or 304) upon the closure of valves 311 (resp. 321 or 331 or 341) and 315 (resp. 325 or 335 or 345) and the opening of valve 318 (resp. 328 or 338 or 348).

The expansion phase is terminated after a predetermined time or upon the attainment of a predetermined pressure which can lie above ambient pressure. The gas liberated by this expansion is substantially free from the strongly adsorbed component and can be discharged as waste gas via line 361 or used to repressurize an adsorber which has previously been desorbed.

In the initial phase, pressure equalization is effected via line 354 between adsorbers 301 (resp. 302 or 303 or 304) and 303 (resp. 304 or 301 or 302) by opening valves 317 (resp. 327 or 337 or 347) and 334 (resp. 344 or 314 or 324). The pressure in absorber 301 (resp. 302 or 303 or 304) is thereby dropped at line t₆ to a pressure P_(R) which as a rule lies well below ambient pressure.

Thereafter, the adsorber 301 (resp. 302 or 303 or 304) can be flushed by a gas which is drawn from the adsorber 304 (resp. 301 or 302 or 303) by the vacuum pump 357.

During flushing the pressure can raise, as has been illustrated, at slightly above ambient pressure, although it is also possible to carry out the flushing at ambient pressure or somewhat thereunder.

During the flushing phase, effluent gas which is, via valve 341 (resp. 311 or 321 or 331), drawn from the adsorber 304 (resp. 301 or 302 or 303), is displaced in part by the vacuum pump 357 past the throttle valve 356 and through line 353 and valve 313 (resp. 323 or 333 or 343) into the adsorber 301 (resp. 302 or 303 or 304).

From the latter flows a gas whose contents in the strongly adsorbable component lies above the contents thereof in the raw gas, this effluent passing via the open valve 316 (resp. 326 or 336 or 346) and line 358 back ahead of the compressor 350 where it is combined with raw gas for a merge to another adsorber in the adsorption cycle.

After flushing, the pressure in adsorber 301 (resp. 302 or 303 or 304) is reduced. To this end, gas is drawn via line 353 and open valve 312 (resp. 322 or 332 or 342) by the vacuum pump 357 from the adsorber 301 (resp. 302 or 303 or 304).

A portion of the product gas, especially during an early part of the evacuation although low is desired over the entire evacuation, is recycled via the throttle valve 356, valve 323 (resp. 333 or 343 or 313) and line 353 for flushing the adsorber 302 (resp. 303 or 304 or 301). The balance is discharged as the product gas.

After termination of the evacuation phase, which is considered identical to the production phase, the pressure rebuilding to the adsorption pressure is carried out in two stages.

In the first stage, we effect a pressure equalization of the adsorber 301 (resp. 302 or 303 or 304) with the adsorber 303 (resp. 304 or 301 or 302) which at that moment is subjected to uniflow expansion. For this purpose valves 314 (resp. 324 or 334 or 344) and 337 (resp. 347 or 317 or 327) are opened. After a predetermined time sufficient to effect pressure equalization, this connection is interrupted. At this point (t₇), the pressure generally is below ambient pressure.

The initial pressure building stage is effected by opening valve 311 (resp. 321 or 331 or 341) utilizing the raw gas for pressurization. A new cycle can then begin.

    ______________________________________                                         TEST DATA                                                                      ______________________________________                                         Adsorption means                                                                              carbon molecular sieve with a BET                                              surface of 1100 m.sup.2 /g                                      charging (adsorption)                                                                         3.00 bar                                                        pressure                                                                       raw gas        16.7% by vol. CO.sub.2                                                         83.3% by vol. N.sub.2                                           adsorber volume                                                                               1 m.sup.3                                                       mode of operation                                                                             version 4                                                       ______________________________________                                         gas volumes (normalized at standard temperature and pressure)                                     CO.sub.2 (m.sup.3)                                                                       N.sub.2 (m.sup.3)                                                                    total gas (m.sup.3)                         ______________________________________                                         1.  pressure buildup                                                                              1.15       5.76  6.91                                           (raw gas)                                                                  2.  adsorption     5.93      29.58 35.51                                           (raw gas)                                                                  3.  flushing       6.68      --     6.68                                           (product gas)                                                              4.  desorption     13.48     --    13.48                                           (product gas)                                                                  product gas yield                                                                             6.80      --     6.80                                           (99.9% by vol. CO.sub.2)                                                       waste gas:     0.28      35.34 35.62                                           (0.8% by vol. CO.sub.2)                                                    yield = (product gas volume) : (raw gas volume) = 0.96                         ______________________________________                                    

    ______________________________________                                         TEST DATA                                                                      ______________________________________                                         Adsorption means    silica gel                                                 charging (adsorption)                                                                              3.00 bar                                                   pressure                                                                       raw gas             16.7% by vol. CO.sub.2                                                         83.3% by vol. N.sub.2                                      adsorber volume     1 m.sup.3                                                  mode of operation   version 4                                                  ______________________________________                                         gas volumes (normalized at standard temperature and pressure)                                     CO.sub.2 (m.sup.3)                                                                       N.sub.2 (m.sup.3)                                                                    total gas (m.sup.3)                         ______________________________________                                         1.  pressure buildup                                                                              0.80       3.98  4.78                                           (raw gas)                                                                  2.  adsorption     5.23      26.09 31.32                                           (raw gas)                                                                  3.  flushing       7.14      --     7.14                                           (product gas)                                                              4.  desorption     12.51     --    12.51                                           (product gas)                                                                  product gas yield                                                                             5.37      --     5.37                                           (99.9% by vol. CO.sub.2)                                                       waste gas:     0.66      30.07 30.73                                           (2.2% by vol. CO.sub.2)                                                    yield = (product gas volume) : (raw gas volume) = 0.89                         ______________________________________                                    

    ______________________________________                                         COMPARATIVE EXAMPLE                                                            (According to German Patent Application                                        P 32 22 560.1 - Example 1)                                                     ______________________________________                                         Adsorption means                                                                              carbon molecular sieve with a BET                                              surface of 1100 m.sup.2 /g                                      charging (adsorption)                                                                         1.2 bar                                                         pressure                                                                       raw gas        16.7% by vol.                                                                  83.3% by vol. N.sub.2                                           adsorber volume                                                                               1 m.sup.3                                                       ______________________________________                                         gas volumes (normalized at standard temperature and pressure)                                     CO.sub.2 (m.sup.3)                                                                       N.sub.2 (m.sup.3)                                                                    total gas (m.sup.3)                         ______________________________________                                         1.  pressure buildup                                                                              0.85       4.25  5.10                                       2.  adsorption     4.09      20.40 24.49                                           (raw gas)                                                                  3.  flushing       18.37     --    18.37                                           (product gas)                                                              4.  desorption     22.50     --    22.50                                           (product gas)                                                                  product gas yield                                                                             4.13      --     4.13                                           (99.9% by vol. CO.sub.2)                                                       waste gas:     0.81      24.65 25.46                                           (2.2% by vol. CO.sub.2)                                                    yield = (product gas volume) : (raw gas volume) = 0.84                         ______________________________________                                     

We claim:
 1. In a method of recovering, in a pressure swing adsorption process, at least one relatively highly adsorbable product gas component for a raw gas containing said product gas component in admixture with at least one less adsorbable gas component in a cycle wherein:(a) in an adsorption phase in a pressure swing adsorber, said raw gas is passed through an adsorber bed at an adsorption pressure in a first gas flow direction to adsorb said highly adsorbable product gas component on said bed and past a waste gas containing said less adsorbable component, the adsorption phase of step (a) being terminated before breakthrough of said highly adsorbable product gas component through said bed; (b) said bed is flushed in a flushing phase with a flushing gas such that at least toward the end of said flushing phase said flushing gas has a concentration of said highly adsorbable product gas component substantially equal to that in a product gas; (c) the pressure is thereafter reduced in said adsorber in a desorption gas to desorb said highly adsorbable product gas component from said adsorber to produce said product gas containing said highly adsorbable product gas component, said product gas being withdrawn from said adsorber, and a portion of said product gas is fed as a flushing gas for use in a subsequent practice of step (b); and (d) the pressure in the adsorber desorbed in step (c) is thereafter rebuilt to said adsorption pressure, the improvement which comprises: (e) intermittently between the end of said adsorption phase and before the commencement of said desorption phase in the cycle of said adsorber, drawing a partial gas flow from said adsorber in the same direction as the flow of gas therethrough during an immediately prior period of the cycle to reduce the pressure in said adsorber; and (f) reusing said partial gas flow in said pressure swing adsorption process.
 2. The improvement defined in claim 1 wherein during flushing in step (b) a gas is recovered initially from said adsorber which is discharged from said waste gas and during a subsequent stage of said flushing phase, a gas is recovered from said adsorber which is subjected to further enrichment in said product gas component by pressure swing adsorption.
 3. The improvement defined in claim 2 wherein substantially 10 to 90% by vol. of said product gas is utilized in step (b) for adsorber flushing.
 4. The improvement defined in claim 2 wherein during substantially the last third of the duration of said flushing phase the resulting gas enriched in said product gas component is fed at a period rising from the end of a desorption phase to the beginning of a flushing phase of a further adsorber operating in parallel with the first mentioned adsorber, to said further adsorber.
 5. The improvement defined in claim 4 wherein the gas evolved during reduction in pressure of said adsorber from the adsorption pressure to the flushing phase pressure is communicated to another adsorber of said pressure swing adsorption process to effect the pressure buildup therein in a corresponding step (d).
 6. The improvement defined in claim 2 wherein the pressure rebuilding in step (d) when the raw gas in a mixture containing at most 15% by vol. carbon dioxide is effected with the raw gas.
 7. The improvement defined in claim 2 wherein the pressure rebuilding in step (d) when the raw gas in a gas mixture containing at most 10% by vol. methane, is effected with the raw gas.
 8. The improvement defined in claim 2 wherein the flushing in step (b) is carried out by passing the flushing gas through said bed in said first gas flow direction.
 9. The improvement defined in claim 2 wherein the desorption step (c) is carried out with withdrawal of said product gas from opposite ends of said adsorber.
 10. The improvement defined in claim 2 wherein the pressure rebuilding in step (d) and the subsequent adsorption phase of step (a) are carried out over a total time interval which ranges between half the duration of one of the others of said phases.
 11. The improvement defined in claim 2 wherein the desorption phase (c) is carried out in two stages, including:a first step (c₁) in which said adsorber is connected to a first vacuum pump reducing the pressure in said adsorber to a pressure from that of flushing in step (b) to an intermediate pressure; and a second stage (c₂) directly following stage (c₁) in which said adsorber is connected to a second vacuum pump reducing the pressure in said adsorber from said intermediate pressure to a final desorption pressure at which said product gas is evacuated from said adsorber by said second vacuum pump.
 12. The improvement defined in claim 11 wherein said intermediate pressure is between 0.1 to 0.95 times the difference between the pressure of said flushing step (b) and said final desorption pressure, and said final desorption pressure is between 0.01 and 0.6 bar.
 13. The improvement defined in claim 12 wherein said intermediate pressure is between 0.4 and 0.8 times said difference and said final desorption pressure is between 0.03 and 0.2 bar.
 14. The improvement defined in claim 2 wherein said flushing phase is carried out at a pressure lower than that of said adsorption phase.
 15. The improvement defined in claim 14 wherein said adsorption phase is carried out at a pressure greater than ambient atmospheric pressure and gas evolved during reduction in pressure to the flushing phase pressure is discharged as waste gas as long as the concentration of said product gas components therein is less than that in said raw gas.
 16. The improvement defined in claim 15 wherein the adsorption phase pressure in said adsorber is in excess of 3 bar.
 17. The improvement defined in claim 15 wherein the gas evolved during reduction in pressure at the flushing phase pressure is fed to a compressor for said raw gas compressing said raw gas to the adsorption phase pressure upon the concentration of said product gas component in said evolved gas exceeding the concentration of said product gas component in said raw gas.
 18. The improvement defined in claim 14 wherein further reduction in pressure in said adsorber for said flushing phase is terminated and flushing is carried out in step (b) at the termination pressure when the product gas component concentration in the gas evolved during the reduction in pressure from the adsorption phase pressure has a value between the mean concentration of said product gas component in said product gas and the concentration of said product gas component in said raw gas.
 19. The improvement defined in claim 2 wherein said process is carried out in said cycle with a multiplicity of adsorbers staggered with respect to phase, each of said adsorbers following the respective adsorption phase being connected to another adsorber which has been evacuated for a respective desorption phase, gas from the adsorption phase adsorber passing in said direction to the evacuated adsorber until the connected adsorber achieve pressure equilibrium.
 20. The improvement defined in claim 2 wherein said process is carried out in said cycle with a multiplicity of adsorbers staggered with respect to phase, each of said adsorbers following the respective flushing phase being connected to another adsorber which has been evacuated for a respective desorption phase, a gas from the flushing phase adsorber passing in said direction to the evacuated adsorber until the connected adsorbers achieve equalization.
 21. The improvement defined in claim 2 wherein said process is carried out in said cycle with a multiplicity of adsorbers staggered with respect to phase, each of said adsorber being connected to another adsorber which has been evacuated for a respective desorption phase until the connected adsorbers achieve pressure equalization, concurrent flow expansion being effected prior to this pressure equalization. 